This invention relates to the coating of narrow linear material such as strip, and especially wire, with metal coatings in a molten metal coating bath. More particularly the invention relates to the combined use of protective atmospheres and gas wiping in treating linear material issuing from a molten metal coating bath in order to establish an accurate and uniform thickness of coating on the surface of the linear material.
Metallic linear material such as strip and wire have been economically coated for many years by passing the linear material through a bath of molten metal, such as molten zinc or aluminum. Usually the linear material has been a ferrous material, such as steel or the like. The coating of aluminum or zinc or sometimes other metals or alloys, such as tin or terne (an alloy of lead with up to 25% tin), provides corrosion resistance to the underlying ferrous metal.
Linear material issuing from a molten metal coating bath usually does not have a satisfactory layer of molten coating metal on its surface. The molten metal coating is invariably either too thick, too uneven, or both, or has some other defect which would prevent the molten metal from solidifying into a satisfactory metal coating upon the substrate metal. As a consequence, it has been customary to wipe the coating in some manner after the linear material leaves the molten coating bath in order to smooth and/or reduce the weight, or thickness, of the coating. Various wiping devices have been used to wipe the coating while it is still molten including soft wipers such as asbestos wipers and the like, rigid wipers such as rolls and scrapers and occasionally semi-rigid wipers composed of layers of various materials such a charcoal or gravel through which the coated linear material passes. More recently gas wipers, or gas doctors, have been used to blow a gas such as air, steam or some inert or reducing gas forcibly against the surface of the molten metal coated linear material to remove excess metal and smooth the coating of molten metal.
In order to attain good adherence of the coating metal to the substrate metal it is necessary for the surface of the substrate to be clean prior to passage through the molten coating bath. The linear material must, therefore, be cleaned prior to being coated to provide a suitable clean, active substrate surface for contact with the molten coating bath. Once the substrate metal is clean it must be kept active, i.e. oxide free, until it is submerged in the molten coating bath. It is therefore necessary to protect the substrate metal after cleaning either with a coating of flux or else by immersion or continuous bathing in an inert or reducing atmosphere. Thus, ferrous linear material frequently enters the molten bath in a protective or oxygen excluding atmosphere of some nature. The protective atmosphere will usually be composed of either an effectively inert gas or a reducing gas or gases.
Inert or reducing atmospheres have also been maintained about the linear material as it exits from the molten bath to prevent excessive or otherwise detrimental oxidation of the surface of the coating while it is still hot, both before and after the coating solidifies. The protective atmosphere is usually contained in a protective chamber, or hood, of some form which extends to or into the surface of the molten bath.
With the more recent frequent use of gas wipers for smoothing and wiping the molten coating, the use of an inert or more frequently a reducing gas to wipe the surface of the linear material has sometimes been adopted to prevent surface oxidation of the coating metal. In some installations, and particularly in wire wiping installations, the wiper has been enclosed in or attached to a chamber containing a protective atmosphere so that the molten coating on the wire is completely protected from exposure to the atmosphere until it is smoothed and wiped.
The use of a non-oxidizing gas as both a wiping and a protective gas has been found to be particularly desirable in the wiping of wire material. Otherwise, oxidized coating particles on the molten coating surface tend to increase the viscosity of the molten metal and result in buildup of a thick viscous oxide coating layer which seriously interferes with effective gas wiping. The small circumference of the wire allows viscous rings of oxide material to form about the wire and break through the gas barrier resulting in thick rings of coating on the wire, which rings crack and flake when the wire is bent after solidification of the coating.
One problem which has been encountered in such combined wiping and protective gas installations as, for example, that illustrated in U.S. Pat. No. 3,707,400, which discloses a combination of a closed hood, containing an inert gas, and a wiping die, that may use the same inert gas as a wiping gas, has been a tendency of the wiping die to provide very poor control of the thickness of the final coating if only the force of the wiping gas is depended upon to establish the thickness of the coating. This has been so in spite of the fact that such combined wiping and protective gas arrangements very efficiently and effectively wipe excess coating from and smooth linear material such as wire passing through the die. However, the exact final thickness of coating has been impossible to control without varying the parameters of the wiping die itself. In other words, while the smoothing of the coating is very effective and a large excess of coating material can be removed from the coated material, actual dimensional control of the coating thickness by the wiping gas has not been satisfactory. It has thus been necessary in many cases to vary the velocity of passage of the linear material through the wiping die in order to effectively control the degree of wiping of molten coating from the surface of the linear material. If the molten coating layer is too thick, it has been necessary to decrease the speed of passage of the linear material through the die orifice in order to decrease the coating layer. If the coating layer is too thin, on the other hand, it has been necessary to increase the speed of the linear material through the die orifice in order to increase the thickness. Naturally, the necessity to adjust the speed of the coating line in order to attain a desired coating weight is undesirable, because such adjustment interferes with other operational and production considerations.
A further problem with prior wiping apparatus and methods particularly in the coating of wire material has been poor concentricity of the final coating with the wire. In a "concentric coating" the coating thickness is substantially equal on all sides of the wire or all around the wire. In a non-concentric coating the thickness of the coating on one or more side of the wire is significantly thicker than the thickness of the coating on the diametrically opposite side or sides. The coating may be concentric on portions of the wire and non-concentric on adjoining portions of the wire. Usually the concentricity varies in a more or less random manner along any given length of wire. In fact it is substantially impossible to obtain a substantial length of perfectly concentric hot dip coated wire particularly with prior known apparatus.
The importance of concentricity is really the avoidance of thin spots in the coating and it will be evident that thin spots may occur because of other factors such as out-of-round or oval coating deposits or the like as well as true non-concentricity. One measure of concentricity then is the number of thin spots in a coating, it being realized that complete concentricity or absence of thin spots is substantially impossible in hot dip coating of wire. It has been the experience of the present inventors that in a hot dipped aluminum-zinc coating, for example, the best or most concentric wire coating which could be obtained using prior gas wipers--based upon an aim coating of 1.5 mils, or thousands of an inch, (i.e. 0.38 millimeter) and with a thin spot defined as any coating area of less than 0.5 mil (0.0127 millimeters) as measured by a commercial type non-destructive spot coating weight detector--would have over 2.5% of the measurements on any given length of wire below 0.5 mil (0.0127 millimeters). In other words, with a thin spot being defined as approximately one third of the aimed for or desired coating thickness, an average of over 2.5% of all tested spots on any given length of wire will turn out to be thin spots, or below minimum in thickness. While this number of thin spots is quite acceptable for most purposes, especially in a sacrificial coating, such as, for example, zinc or aluminum-zinc on a ferrous base material, it does represent a waste of coating metal since somewhat thicker coatings must be used on all portions of the wire to prevent an excess number of unacceptably thin spots.